The present invention relates to the press section of a paper making machine and particularly a press jacket for a press device which has a press shoe. During operation of the press device, the press jacket rotates and slides over the press shoe. The press shoe presses the press jacket against a backing roll. The slide surface of the press shoe is usually of concave shape producing a press nip which is lengthened in the circumferential direction between the press jacket and the backing roll. The press jacket can be tubular in shape and be closed at both its lateral ends. As used below, the expression press jacket also includes an endless, laterally open press belt which also forms a lengthened press nip between a press shoe and a backing roll. Outside the press nip, the press belt travels preferably over guide rolls.
The invention proceeds from a press jacket having features which are known from German DE 40 22 800 Cl which corresponds to U.S. Pat. 5,118,391.
In shoe presses, frictional heat is produced in the lubricant layer, usually an oil film, that is disposed between the rotating press jacket or press belt and the stationary press shoe. This increases the temperature of the lubricant and thereby also of the press jacket. The friction generated heat is removed from the press zone along with the lubricant. The heat must be removed from the lubricant before the press jacket reenters the press zone. The smaller the amount of lubricant that is conveyed along with the press belt, the more that its temperature increases. This presents a danger and the life of the press jacket will be reduced. There are also greater temperature differences over the width of the press jacket if the thickness of the lubricant film differs.
The invention therefore concerns conveying as much lubricating oil as possible through the nip between the press jacket and the press shoe. This increases the life of the press jacket. Possible temperature differences over the width of the press jacket are reduced to a minimum.
U.S. Pat. No. 4,482,430 discloses a press belt having an inner or shoe facing surface in which recesses for the transport of the lubricant are cast or are mechanically worked. The recesses are of considerable depth, having an order of magnitude of several millimeters. The material of the belt is elastically deformable, so that the recesses and their sidewalls are flattened in operation by the action of pressure. The disadvantages of this known solution are:
a) its high cost of manufacture; b) the considerable thickness of the press belt and thus its high flexural stiffness in the circumferential direction as well as in the longitudinal direction which leads to extensive wear of the press belt, particularly upon the three dimensional curvature at the ends of the shoe; c) due to the considerable depth of the recesses, there is a danger that turbulences may be formed in the layer of lubricant, which would cause increased friction. The result is that a relatively high drive power is necessary for such a press device so that additional frictional heat is produced; d) the compressibility of the material of the belt results in a relatively low strength and a relatively low modulus of elasticity, and therefore there is relatively little resistance to elongation of the belt; and e) finally, due to the considerable depth of the recesses, there is a relatively high circumferential tension at the lowest point or bottom of each recess and cracks can result here in case of too great stressing.
The known system could theoretically operate at lower speed, namely if the press shoe were of infinite length. In practice, however, the press shoe has a finite length. The following problems arise at the end of the press shoe. With continuous deep grooves extending parallel to the belt axis, the layer of lubricant is destroyed since lubricant can flow off laterally off the belt. With pocket-like depressions, there is a danger of the bars or ridges present between the pockets being squeezed out laterally at the ends of the press shoe so that the inside of the press jacket is destroyed within a relatively short time in the regions of the ends of the press shoe.